1. Field of the Invention
This invention relates to rotary extruders, and more particularly, to rotary extruders having an integral auger feed mechanism.
2. Description of the Prior Art
Rotary extruders are well known, having previously been disclosed, for example, in U.S. Pat. Nos. 3,880,564; 4,012,477; 4,501,543; 4,813,863; and 4,887,907. As used herein, the term "rotary extruder" refers to an extruder having a barrel section with an eccentric bore and a rotor rotatably mounted within the bore that comprises a substantially cylindrical plasticizing surface.
Unlike conventional screw-type extruders, which are typically "flood fed", rotary extruders have heretofore been operated in a "starve fed" manner. In the rotary extruders previously known, the feed material, preferably comprising a major portion of a polymeric resin, is fed downwardly through a feed port by gravity flow into an annulus defined by the plasticizing surface of cylindrical rotor and the inwardly facing surface of the surrounding barrel. The polymer is carried into the annulus from the feed port by the rotational motion of the rotor. A thin coating of plasticized polymer on the rotor surface aids in carrying the unplasticized polymer into the annulus.
Although the rotary extruders previously disclosed have been satisfactorily used for extruding many polymeric materials, difficulties have been encountered when attempting to increase feed rates. Efforts to increase the feed rate in conventional rotary extruders have sometimes led to clumping or polymer buildup in the feed section, often caused by polymer sticking to hot surfaces around the feed port. This can in turn cause surging and thermal degradation. Attempts to avoid clumping or sticking of feed material inside the feed port through use of a cooled insert in the feed port have also met with difficulties due to the intricate geometries involved, resulting thermal expansion differences, and other polymer feed restrictions. Enlarging the feed port of conventional rotary extruders to increase the flow area can cause undesirable weakening of the barrel.
Difficulties have also been encountered in venting gases when feeding low bulk density or "fluffy" materials into conventional rotary extruders in that the incoming materials tend to be suspended by the exiting gases. Other problems have been encountered in feeding non freeflowing materials such as additives, some pigments and the like that are likely to clump in the feed port.
The sequential use of conventional screw-type extruders in two-stage extrusion processes is well known, being disclosed, for example, in U.S. Pat. No. 4,863,653. The use of augers for controlling the rate at which material is supplied to the feed ports of conventional extruders for gravitational feeding is also known, being disclosed for example in U.S. Pat. No. 4,022,440.
U.S. Pat. No. 4,124,307 discloses a homogenizer for composite viscous materials in which a highly viscous mass to be homogenized is fed from a screw plasticizer into a mixing chamber perpendicular to the plasticizer mouth. A mandrel rotates independently of the plasticizer screw in the plasticizer mouth to masticate the mass in a narrow annular clearance between itself and the chamber wall.